Haemodialysis treatments have been developed since the early 1960s using a variety of combinations and arrangements of catheters. The earliest treatments were conducted using two needles in the same vein and this subsequently led to pioneer work done by Dr. Shaldon in England who used two flexible catheters which could be left in place for limited periods. Some practitioners proposed the use of a dual flow catheter because this could be entered through a single incision rather than two. From this two basic types were developed. One was a coaxial catheter with the intake lumen surrounding the return lumen, and the other a catheter having side-by-side lumens either in individual tubes connected to one another or in a single extrusion defining two lumens.
Catheters having side-by-side lumens have the disadvantages that because the lumens are side-by-side, the intake openings can be in one side of the catheter only. As a consequence of this, if the catheter were to attach itself to the wall of a vein due to suction applied to the intake lumen, then of course the flow would stop. Medical staff then have to move the catheter by rotating it until blood again flows. This is a very delicate manipulation which is normally performed only by a qualified medical practitioner who must be available at all times in case the flow is discontinued. This disadvantage has resulted in renewed interest in coaxial devices which can be made to have intake openings in any part of the wall of the catheter. As a result, no matter where the catheter may rest against a vein, some of the intake openings remain patent. There is then less likelihood that during use the catheter must be serviced by a trained medical practitioner.
Coaxial catheters are subject to design criteria which can be difficult to meet and which may have contributed to the initial popularity of side-by-side structures. Because the coaxial catheter is inherently less resistant to kinking than side-by-side structures, the tubes used in the structure have had relatively thick walls with the result that the catheter had to also have a larger cross-sectional area than an equivalent side-by-side catheter. One of the reasons for this is that typically the inner tube projected beyond the outer tube to form a distal tip section. Because of this the cross-section of the inner tube has to be chosen to have sufficient rigidity to permit engagement over a Seldinger wire. This consideration set the size of the inner tube.
Another approach is taught by U.S. Pat. No. 4,493,696 to Uldall. The catheter shown in this patent was designed specifically to permit removal of the inner tube between treatments. It was thought at the time of the patent (i.e. about 1980) that such a procedure would be desirable. However, it is now accepted that with modern techniques it is not necessary to remove the inner tube. Also, the inherent disadvantages of accidental separating of the tubes, accurate location of one tube relative to the other, sizing, and the sudden change of section at the end of the inner tube, were all obstacles to the use of this catheter.
The present catheter uses a structure which permits a relatively thin walled inner tube to be used to minimize the overall cross-section of the catheter. Also, the inner tube meets the tip section smoothly to provide a continuous inner lumen at a transition portion where the inner and outer tube materials meet in a permanent bond to minimize the risk of accidental separation and to ensure repeatable manufacturing methods for more constant catheter structures.